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[DIY contact mics]
> You buy a piezo buzzer.
> and carefully break off the black hard plastic casing (I use needle nose 
> pliers) exposing the inner wafer.
> Then solder to leads to each of the leads coming off the wafer and 
>attach 
> them to a female 1/4"  jack.
Piezos are rather high-ohmic, with the resistance being a complex function 
of displacement. To make that work (especially with regard to minimize EMI 
and have acceptable sound quality when driving slightly longer lines), 
include a transformer in your mike. One of those which are used for a DI 
or for a high-impedance microphone - one side is fed by the microphone, 
the other one goes out symmetrically to an XLR.
To save you the DIY part here, get Behringer's DI20 (active 2-ch DI box 
for around €20), which you would obviously keep as close to the mic as 
possible.
There, I used the B-word again ;)
           Rainer